1. Field of the Invention
The present invention relates generally to the field of electromechanical devices, and more particularly, to a linear variable differential transformer with complimentary step-winding secondary coils.
2. Description of the Related Art
Linear variable differential transformers (LVDTs) are electromechanical transducers that convert rectilinear motions of externally coupled objects into electrical signals that are proportional to the positions of the objects. LVDTs have been utilized in conjunction with a wide range of measurement and control devices such as flowmeters, strain gages, and pressure sensors. Important characteristics of a practical LVDT include (i) the ability to produce a linear output signal over a relatively large displacement range relative to the overall length of the device, (ii) durability and reliability, and (iii) relatively low cost of manufacture.
Numerous techniques have been employed in the prior art to maximize the linear operating range of LVDTs. Prior art devices are relatively difficult to construct and are therefore relatively expensive to manufacture. The current invention provides a highly linear output signal over a long range of rectilinear displacement, while being simpler and less expensive to construct than currently available LVDT devices.
U.S. Pat. No. 3,054,976 (Lipshutz, 1962) is one such example of a prior art device. In Lipshutz's first embodiment, two separate and overlapping secondary coil windings are utilized. The windings are tapered, with a maximum number of wire layers applied at one end of coil, and the number of layers gradually decreasing to zero at the other end of the coil. The coils are connected with opposing polarities, so that the induced voltages are opposite in phase, causing the net output from the device to be the difference between the induced voltages in the two secondary windings. When a ferrous core is moved longitudinally within the coil array, the induced voltage in one coil increases while the induced voltage in the other coil decreases, thereby producing a variable output voltage that is linearly proportional to the core's position. There are two practical disadvantages to constructing this embodiment of LVDT. The first disadvantage (noted in Lipshutz's patent) is that the core is located physically closer to one of the secondary windings than the other. This arrangement causes the “null point” (i.e., the zero output voltage point) of the core to be physically off-center from the center point of the two secondary coils, which in turn potentially adversely affects the linear range (or maximum measurement range) of the device for some applications. The second—and more important—disadvantage of this embodiment is the difficulty of manufacturing tapered windings by machine or manually. It is impractical to produce coils having sufficient taper with the wire diameters required for LVDTs in practical sizes. Currently available commercial LVDTs do not utilize tapered windings, due to the difficulty of manufacturing such devices.
Lipshutz's second embodiment describes an alternate and more symmetrical tapered winding configuration for the secondary coils that overcomes the null point problem but does not address the difficulty of practical manufacture of the tapered windings.
Lipshutz's third embodiment replaces the tapered windings with a series of discrete multiple coils that are wound between raised ribs or fins on the coil form. Currently available commercial LVDTs use a modified form of this embodiment, in which the coils are pre-wound on individual bobbins, which are then slipped over the coil form and electrically connected. A typical LVDT similar to the one depicted by Lipshutz utilizes 7 primary and 8 secondary coils, which require approximately 34 internal electrical connections. These connections require additional manufacturing steps and are a potential source of reduced reliability for the device.
Lipshutz also describes an embodiment “in which the number of turns in the windings of a differential transformer involving tapered windings may be calculated and wound quickly and efficiently.” This embodiment was not intended to be used as an actual LVDT but rather as a prototyping tool for determining the correct winding arrangement for building tapered-coil LVDTs. This embodiment comprises multiple secondary coils (16 of each in the patent illustration), in which the first and second secondary coils are wrapped with one on top of the other. Construction of this device requires winding a portion of the first secondary coils, then winding a portion of the second secondary coils, then again winding a portion of the first secondary coils, until all of the multiple coils are completed. There are no edge supports (i.e., ribs) to keep the individual coils separated and properly stacked in this device, and Lipshutz does not describe the methods used to prevent the relatively short, thick coils from collapsing during the winding process without edge supports.
U.S. Pat. No. 5,327,789 (Nijdam, 1994) is another example of an attempt to address the deficiencies in traditional LVDTs. This patent describes a combined flowmeter-LVDT device that comprises magnetic cores, magnetic coils, and electronics to produce an electrical signal that is proportional to displacement of a magnet within the coil form. This device utilizes relatively simple coils in combination with relatively complex electronic circuitry to produce the desired proportional output. This invention is limited in that the output from the coils are nonlinear; in other words, the output voltage does not vary linearly with the displacement of the core. The non-linear output signal is converted to an equivalent flow rate by use of a “look-up table” that is stored in the electronic circuitry.
U.S. Pat. No. 5,061,896 (Schmidt, 1991) describes a variable transformer with a moveable core and multiple coils that has an output proportional to displacement of the core. In this device, the output voltage remains constant, while the phase angle of the output signal varies proportionally to the displacement of the core. This device utilizes variable-pitch coils and requires relatively complex electronic circuitry, all of which adds to the manufacturing cost.
U.S. Pat. No. 4,134,065 (Bauer et al., 1979) describes an LVDT embodiment that comprises two primary coils and one secondary coil. In this device, displacement of the moveable core is proportional to the phase (not amplitude) of the secondary coil output voltage. In other words, the peak amplitude of the secondary coil remains constant as the inner core moves, while the phase of the output signal changes. As an example, assume that the output voltage is an alternating current sine wave. As the core moves within the LVDT, the peak voltage (i.e., the maximum voltage at the top of each sine peak) of the output does not vary; however, the sine wave shifts left or right (as a function of time) as the core moves. This effect is known as “phase shift” or “variable phase.” The magnitude of a sine wave is not linearly proportional to phase shift. Because a linear phase shift of a sine wave results in a non-linear change in output signal, the output of the Bauer device is “linearized” by use of non-uniform layers of coil wraps in primary coils. The thickness of the primary coils is greatest at each end of the mandrel and decreases to minimum thickness at each end of the mandrel. In order to vary the thickness of the primary coil wrappings, Bauer varies the number of layers in the coil wrapping, and he uses discrete “steps” of variation, as opposed to a gradual taper.
In contrast to the present invention, Bauer uses stepped wraps on the primary coils, whereas the present invention comprises stepped wraps on the secondary coils. Bauer uses a constant-thickness secondary coil layer, whereas the present invention utilizes a constant-thickness primary coil layer. Bauer uses two primary coils and one secondary coil, whereas the present invention comprises one primary and two secondary coils. Furthermore, the step size in the Bauer device is not constant; it varies so as to form a non-linear change in coil thickness over the length of the mandrel. This variation in step length is required in the Bauer invention to produce the desired linear relationship between core displacement and output signal. Perhaps most significantly, the present invention is much simpler to construct than the Bauer device. The tall stack comprised of multiple short layers in the Bauer device would be difficult to construct without having the layers collapse during manufacture.
Accordingly, it is an object of the present invention to provide an improved LVDT in which voltage magnitude is used to track core displacement, the output voltage is linearly proportional to core displacement, and the linear operating range of the LVDT is maximized in relation to the overall length of the device. It is a further another object of the present invention to provide an LVDT that is highly reliable, simpler to construct and less expensive to manufacture than prior art devices.